When it feels like you’re writing some external udf thats executed in another environment, it does not feel as nice as throwing in a lambda, even if the lambda is not ideal.
https://docs.pola.rs/api/python/dev/reference/expressions/ap...
You can also iter_rows into a lambda if you really want to.
https://docs.pola.rs/api/python/stable/reference/dataframe/a...
Personally I find it extremely rare that I need to do this given Polars expressions are so comprehensive, including when.then.otherwise when all else fails.
It also does batches when you declare scalar outputs, but you can't control the batch size, which usually isn't an issue, but I've run into situations where it is.